Spectroscopic and imaging studies of materials are of continuing interest for their role in helping to understand fundamental structures. For imaging and analyzing highly localized structures, however, conventional optical instruments are not suited since they operate under the classical diffraction limit. Recent advances have demonstrated the ability to perform spectroscopic analysis on single atoms or molecules confined in various electromagnetic and optical traps. The ability to image existing quantum sites in a solid could mean a significant advance in understanding of the quantum nature of materials.
Non-optical techniques, for the study of solids have long been available. Electron microscopy achieves nanometer resolution, but requires a vacuum environment and a generally conductive sample, frequently requiring the deposition of an external metallic layer onto the sample. More recent physical scanning microscopes, the scanning tunneling microscope (STM), the atomic force microscope (AFM) and the lateral force microscope (LFM) are all able to image surfaces with exceptional resolution using the control of a fine probe interacting with the surface of a sample.
Since the advent of the STM, a new class of related optical measurements has begun to emerge. These Scanning Optical Microscopy (SOM) techniques can be divided into two categories: (1) Near Field Scanning Optical Microscopy (NSOM); and (2) Photon Scanning Tunneling Microscopy (PSTM), also referred to as Evanescent Scanning Optical Microscopy (ESOM). Both approaches are intended to provide optical imaging resolution below the classical diffraction barrier and have the capability of providing imaged spectroscopy on a scale not previously realized.
The NSOM and PSTM systems are differentiated primarily in their illumination scheme. Numerous specific geometries have been evaluated. The desired resolution in NSOM is achieved through either control of illumination or collection optics. One approach utilizes scanned aperture illumination. An aperture with a radius of tens of nanometers is placed between the source and the sample and defines the structure of the near field at the sample surface. A microscope objective on the opposite side of the sample collects the transmitted light as the aperture is scanned. Alternatively, collection may be through the scanning of a fine probe in close proximity to the test sample. The scanning collection probe used to regulate resolution admits a wider range of lighting options and is used equally effectively in PSTM.
Within this class of scanning collector techniques resolution is defined by the geometry of a dielectric probe which converts energy from the field at the sample surface into propagating energy within the collector. Analytical studies of NSOM fields and some discussion of scattering mechanisms which may attribute to the tunneling between an evanescent field and a probe have been published. Preliminary experimental work has been conducted which claims that high resolution of at least .lambda./10 is achievable in both techniques.
However, difficulty in the previous work exists in verification of achieved resolution and the relationship between the collector probe and resolution. Efforts on probe development concentrate on developing a tip generally as small as possible, and adding a metal cladding to assist in rejection of stray light as well as providing enhanced field confinement. However, with no discussion of the impact of probe tip design on the achieved resolution, the manufacture of probes and their use in SOM remains a "black art". Additionally the prior choice of nominally single-mode transmission grade fiber as a probe has no impact on resolution and dictates a serious penalty in signal-to-noise ratio (SNR). The result is that the use of the tapered fiber optic probe as a signal collector in near field and evanescent imaging has been largely curtailed, limiting unnecessarily the options to the designer of the SOM systems.
The improved probe of the present invention provides design parameters to manufacture a tapered fiber optic collector probe with resolution below standard diffraction limits and collection efficiencies two orders of magnitude above state-of-the-art fiber optic collector probes.